Motor overloads

Lucas Organ

Member
Location
NL CANADA
Occupation
Industrial Electrician
If I have a motor overload relay sat at 25A and I exceed this by say even just one amp, how long should my OL relay read this current before it trips?

Thank you all in advance
 

Eddy_Current

Member
Location
Canada
If I have a motor overload relay sat at 25A and I exceed this by say even just one amp, how long should my OL relay read this current before it trips?

Thank you all in advance
Depends on the brand but many overloads will carry 125% of what they are set at.

Our code tells us we can set overloads up to 115% or 125% of the motors FLA depending on the service factor of the motor, but many manufacturers documentation will tell you to set the overload to the FLA because they have that extra percentage built in.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
A melting alloy/bimetallic NEMA style overload will usually be able to handle 6X for 20 sec, while an electronic or IEC bimetal is often good for 6X at 10sec.
 

kwired

Electron manager
Location
NE Nebraska
As mentioned already that 25 amps on the dial probably already has 1.15-1.25 factored into it so actual trip curve starts at 28-31.

Should you have a motor with a SF or 1.0 instructions likely tell you to adjust the setting to (I think) 90% of motor nameplate amps. so in that case you should be setting it to 22.5 amps.

And yes some those overload relays would be hard to set to a definite level, you just get close to what the setting is supposed to be.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Isn't that the difference between a Class 20 & 10?
Yes it is.

These devices have inverse time characteristics, so at a small overload, like the OP's 26 amps on a 25A relay, tripping time would be measured many minutes if not longer.
At low levels there is quite a bit of slop in their low level performance, so the manufacturers usually recommend against trying to field test test them.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I can recall back in the good old days of NEMA starters with the heater charts. Some manufactures figured in 125% and others did not. I think it was Square D that had a chart that was for motors with a SF of 1.0 and another chart for those with a SF of 1.1 or higher. You sometimes had to really look at the notes accompanying the charts.
 

paulengr

Senior Member
The class in NEMA terms is the number of seconds before it trips at 600% of FLA, give or take 10% with a max of 125%.At 200% the spec is something like trip in at least 12 minutes with very little lower limit. Even on thermal mag breakers I often see ranges like a Sq D the other day tripped in 6-31 seconds at 600%.

The charts for eutectic overload heaters just got you to a part number. What happened behind the curtain was not something you were supposed to look at. Just use the correct model.

When especially electronic overloads came out they were viewed with suspicion. So to avoid "nuisance tripping" because it wasn't the normal sloppy specs they went for the upper limit.



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kwired

Electron manager
Location
NE Nebraska
I can recall back in the good old days of NEMA starters with the heater charts. Some manufactures figured in 125% and others did not. I think it was Square D that had a chart that was for motors with a SF of 1.0 and another chart for those with a SF of 1.1 or higher. You sometimes had to really look at the notes accompanying the charts.
If you purchase an "open" starter it will usually come with more than one table, you use the one that applies to your application.

If you purchase a starter in an enclosure, the table that applies to that enclosure is already attached to the enclosure.

Either case they will usually have a note at bottom telling you to multiply motor current by a certain factor for motors with SF of 1.0.

If you looking up thermal overload tables in the catalog (this for Square D and many others) you need to know exactly what starter you have, what it will be enclosed in/with, how many thermal units it will have installed, maybe another detail or two in some cases just to select the right table to use.
 
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